->gd is libgd's own internal format. It is unlikely to be recognised by any other graphics libraries or image converters.

It is a very simple format that consists of a 11 byte header (for truecolor) and the X-width * 4 * Y-height bytes of uncompressed image data.

A simple 10x10 (truecolor) image with a diagonal line of colored pixels constructed like this:

$i = GD::Image->new( 10, 10, 1 );;
$i->setPixel( $_, $_, $_ ) for 0 .. 9;;
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And then output as ->gd and unpacked as bytes :

$x = $i->gd;;
print unpack 'C*', $x;;
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looks like this (manully formatted):

255 254            ## (A sig of sorts) Low bit of second byte means tr
+ue color
0 10               ## Width in pixels
0 10               ## height in pixels
1                  ## Indicates no color table (Ie. a non-palette imag
+e)
255 255 255 255    ## Seems to be a trailer block
0000 0000 0000 0000 0000 0000 0000 0000 0000 0000  ## X x Y x 4 bytes 
+(U32s) of rgba pixel data
0000 0001 0000 0000 0000 0000 0000 0000 0000 0000 
0000 0000 0002 0000 0000 0000 0000 0000 0000 0000 
0000 0000 0000 0003 0000 0000 0000 0000 0000 0000 
0000 0000 0000 0000 0004 0000 0000 0000 0000 0000 
0000 0000 0000 0000 0000 0005 0000 0000 0000 0000 
0000 0000 0000 0000 0000 0000 0006 0000 0000 0000 
0000 0000 0000 0000 0000 0000 0000 0007 0000 0000 
0000 0000 0000 0000 0000 0000 0000 0000 0008 0000 
0000 0000 0000 0000 0000 0000 0000 0000 0000 0009
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It should not be hard to knock up Perl script that converts that to one of the ASCII image formats prevalent on *nix, and then convert that to png.

Note:Palleted images have two extra bytes in the header (not sure about the purpose); a 256 * 4-byte color table following the trailer block; and then width * 1-byte * height image data, where each byte value is an index into the color table.